gdb/doc: make use of group/end group in 'info pretty-printers' example

The 'info pretty-printers' example is pretty long and consists of many
commands and their output.

Currently, when the pdf manual is generated this example spans a
page-break, with the page-break falling part way through some example
output from GDB.

This commit breaks up the example using @group .... @end group, within
each group is a single GDB command and all its output.

Now, when the pdf manual is created, the page-break is placed after
the output of one GDB command, and before the subsequent command, this
looks much nicer.

gdb/doc/gdb.texinfo

@@ -12075,6 +12075,7 @@ another from library2.so named @code{bar} that prints two types of objects,
 @code{bar1} and @code{bar2}.
 
 @smallexample
+@group
 (gdb) info pretty-printer
 library1.so:
   foo
@@ -12082,11 +12083,15 @@ library2.so:
   bar
     bar1
     bar2
+@end group
+@group
 (gdb) info pretty-printer library2
 library2.so:
   bar
     bar1
     bar2
+@end group
+@group
 (gdb) disable pretty-printer library1
 1 printer disabled
 2 of 3 printers enabled
@@ -12097,6 +12102,8 @@ library2.so:
   bar
     bar1
     bar2
+@end group
+@group
 (gdb) disable pretty-printer library2 bar;bar1
 1 printer disabled
 1 of 3 printers enabled
@@ -12105,6 +12112,8 @@ library2.so:
   bar
     bar1 [disabled]
     bar2
+@end group
+@group
 (gdb) disable pretty-printer library2 bar
 1 printer disabled
 0 of 3 printers enabled
@@ -12115,6 +12124,7 @@ library2.so:
   bar [disabled]
     bar1 [disabled]
     bar2
+@end group
 @end smallexample
 
 Note that for @code{bar} the entire printer can be disabled,